In integrated optical circuits, a silicon waveguide structure typically comprises a rib formed in the upper epitaxial silicon layer of a silicon-on-insulator chip. The rib has a top surface and side walls, and has trough portions on either side of it. The rib serves to confine an optical transmission mode for light which is contained in the rib and under the trough portions.
It is often desirable to modify the basic waveguide structure to perform a number of different functions. During these modifications, it is frequently required to treat the top surface of the rib in a manner differently to that of the side walls.